Whether listening to an MP3 player while traveling, or to a high-fidelity stereo system at home, consumers are increasingly choosing intra-canal and intra-concha earphones for their listening pleasure. Both types of electro-acoustic transducer devices have a relatively low profile housing that contains a receiver or driver (an earpiece speaker). The low profile housing provides convenience for the wearer, while also providing very good sound quality.
These devices, however, do not have sufficient space to house high fidelity speakers. This is also true for portable personal computers such as laptop, notebook, and tablet computers, and, to a lesser extent, desktop personal computers with built-in speakers. Such devices typically require speaker enclosures or boxes that have a relatively low rise (e.g., height as defined along the z-axis) and small back volume, as compared to, for instance, stand alone high fidelity speakers and dedicated digital music systems for handheld media players.
The drivers (earpiece speakers) for such devices therefore typically use a low profile diaphragm assembly, which is composed of two parts. Namely, a sound radiating surface (SRS) and a suspension member. The SRS vibrates axially thereby creating pressure waves outside the driver enclosure. The suspension surrounds and suspends the SRS within the enclosure and allows it to vibrate axially. Each of these moving parts, however, have natural structural resonances that can be excited at certain frequencies, which are typically different from one another. As a result, at certain frequencies (the so-called “breakup mode” frequency) portions of the SRS (e.g., the inner portion and the outer portion), and in some cases the suspension member, may move out of phase with one another. In other words, in the case of the SRS, the center or inner portion of the SRS may be moving up while the outer portion or edges of the SRS may be moving down. Such out of phase movements, result in an undesirable sound pressure output (e.g., drop in pressure) at the breakup frequency. One way in which breakup modes have been addressed is to increase the stiffness of the SRS, such as by using a stiffer SRS material or making the SRS thicker. In some cases, however, there are manufacturing constraints and/or undesirable performance trade-offs that come along with a stiffer SRS, and therefore this may not be an option.